Method and apparatus for controlling a temperature-controlled probe

ABSTRACT

A method and apparatus to control a power output of a probe connected to a controller in a thermal energy controller system to maintain a target temperature. The system includes a probe, a controller/generator and a means for connecting the probe to the controller. The probe has a thermal element and a temperature sensor. The temperature sensor provides a sensed temperature. The method and apparatus allow the controller to more effectively accommodate different types of probes by providing selectable probe settings for the probes. The controller modifies its operation in response to the selected probe setting. In this way, the power output of each type of probe can be more effectively controlled to better maintain the selected target temperature.

BRIEF DESCRIPTION OF THE INVENTION

[0001] This invention relates generally to medical devices. Moreparticularly, this invention relates to a method and apparatus forcontrolling the temperature of a probe that is used to vary the thermalenergy delivered to tissue during a surgical procedure.

BACKGROUND OF THE INVENTION

[0002] Soft tissue is the most abundant tissue in the human body. Mostsoft tissue is collagen—over 90% of the organic matter in tendons andligaments is collagen. The connective tissue in joints is comprised ofsoft tissue, generally collagen tissue. When soft tissue in a joint isdamaged, the healing process is often long and painful.

[0003] Well-known methods for addressing the treatment of soft tissue ininjured joints include strengthening exercises, open surgery, andarthroscopic techniques. Using current treatments, many people withinjured joints suffer from prolonged pain, loss of motion, nerve injury,and some develop osteoarthritis. The soft tissue in many injured jointsnever heals enough to return the damaged joint to its full range offunction.

[0004] It is known in the art that thermal energy applied to softtissue, such as collagen tissue, in joints may alter or manipulate thetissue to provide a therapeutic response during thermal therapy. Inparticular, applying controlled thermal energy to soft tissue in aninjured joint can cause the collagenous tissue to shrink, therebytightening unstable joints.

[0005] Medical probes for the rehabilitative thermal treatment of softtissues are known in the art. Examples of these probes include laserprobes and RF heated probes. While these tools meet the basic need forrehabilitative thermal treatment of soft tissues, such as collagentissues, many suffer from temperature over-shoot and under-shootfluctuation causing unpredictable results in the thermal alteration.

[0006] One medical probe in U.S. Pat. No. 5,458,596 to Lax, et al.,discloses examples of a probe with a proximal and distal end thatemploys heat for the controlled contraction of soft tissue. However, apotential drawback of many prior art probes is that the probe'stemperature can become unstable when heat from the probe is dissipatedinto the mass of the treated tissue. This situation can be a particularproblem when treating dense tissue; dense tissue acts as a heat sinkthereby requiring additional energy input to maintain the desiredtemperature. The application of additional energy in an attempt tocompensate for the heat sink effect can cause an underdamped effectbefore settling out at the correct temperature.

[0007] In general, a system is overdamped when its damping factor isgreater than one and has a slow response time. A system is criticallydamped when its damping factor is exactly one. A system is underdampedwhen its damping factor is less than one. In an underdamped system,“ringing” is a problem because it can cause the momentary application oftemperatures that are too high for the safe heating of soft tissue. Whenthis occurs, damage to the soft tissue may result from charring,ablation or the introduction of unwanted and harmful effects on the softtissue causing injury.

[0008] Typically, the medical probes are attached to a controller tocontrol the power output of the probe based on an actual temperaturemeasurement from a temperature sensor such as a thermocouple in theprobe and a set predetermined target temperature. The controller is partof a system that includes circuitry to monitor sensed temperature fromthe temperature sensor. Temperature-controlled probes are designed toprovide precise coagulation to eliminate damage, charring, and bubbles.Different size probes with various configurations are available to treatvarious joint sizes including the shoulder, knee, ankle, wrist and theelbow.

[0009] Precise temperature control of the system in which the probes areused is required during various types of thermal therapy of soft tissue.For example, during hyperthermia which is defined as the treatment ofdiseased soft tissue by raising the bodily temperature by physicalmeans, some prior art probes have difficulty in providing smooth andconsistent heating because the preferred materials for the energydelivery electrodes are highly thermally responsive materials. Suchmaterials generally do not retain large amounts of heat energy. Atinitiation, the controller rapidly heats the probe to achieve the targettemperature which can result in an overshoot problem. Duringapplication, contact with large tissue masses tends to cause underdampedfluctuations in the probe temperature due to vast differences in thetemperature of the surrounding tissue mass. Likewise, one skilled in theart will appreciate that similar problems may occur during a desiredreduction in the soft tissue temperature.

[0010] In addition, different probes have different operatingcharacteristics. Applications using larger probes typically needrelatively large amounts of power to reach and maintain the desiredtemperature. Applications using smaller probes, such as a spine probe,need a well-controlled and precise stable temperature. However, thetypical controller uses the same method to control the power output ofall the different probes and does not change its control process inresponse to different types of probes further contributing to overshootand undershoot problems.

[0011] Therefore, a method and apparatus are needed that allows thecontroller to change operation in response to the type of probeattached. This method and apparatus should also reduce temperatureovershoot and oscillation while initiating and applying treatment.

SUMMARY OF THE INVENTION

[0012] A method and apparatus control the power output to a probe tomaintain a target temperature. The probe is part of a system including ameans for connecting a probe to a controller. The probe has a thermalelement and a temperature sensor. The temperature sensor provides asensed temperature. The method and apparatus allows the system and thecontroller to more effectively accommodate different types of probes byproviding at least one selectable probe setting for the probes. Thecontroller modifies its operation in response to the selected probesetting. In this way, the power output of the probe is more effectivelycontrolled to maintain the target temperature.

[0013] A memory provides at least one set of probe settings. The setincludes at least one gain parameter and corresponds to predeterminedoperating characteristics for a probe. A target temperature is received.A first probe setting corresponding to a desired set of operatingcharacteristics for a probe is also received. A set of probe settings isselected in response to the first probe setting. An error signal isgenerated by comparing the sensed temperature to the target temperature.An output control signal is determined by applying a control function tothe error signal. The control function uses the gain parameter from theselected set of probe settings. An amount of power is supplied to thethermal element in response to the output control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] For a better understanding of the invention, reference should bemade to the following detailed description taken in conjunction with theaccompanying drawings, in which:

[0015]FIG. 1 illustrates a controller and probe in accordance with anembodiment of the present invention;

[0016]FIG. 2 illustrates the controller of FIG. 1 in accordance with anembodiment of the present invention;

[0017]FIG. 3 illustrates an exemplary table, stored in the memory ofFIG. 2, associating a particular probe setting with a particular switchposition;

[0018]FIG. 4 illustrates one embodiment of aproportional-integral-derivative (PID) control function of the presentinvention;

[0019]FIG. 5 illustrates an embodiment of the derivative operation ofFIG. 4;

[0020]FIG. 6 illustrates a second embodiment of a PID control functionof the present invention;

[0021]FIG. 7 illustrates a third embodiment of a PID control function ofthe present invention;

[0022]FIG. 8 is a flowchart of the PID control function of FIG. 4;

[0023]FIG. 9 is a flowchart of the derivative operation of FIG. 5 thatis used in step 128 of FIG. 8;

[0024]FIG. 10 is a flowchart of an antiwindup function;

[0025]FIG. 11 is a flowchart of an alternate embodiment of an antiwindupfunction;

[0026]FIG. 12 is a flowchart of an embodiment which varies the targettemperature to reach the final target temperature;

[0027]FIG. 13 is an exemplary temperature profile stored in the memoryof FIG. 2; and

[0028]FIG. 14 is a detailed flowchart of step 188 of FIG. 13.

[0029] Like reference numerals refer to corresponding parts throughoutthe drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0030] In FIG. 1, a probe 16 is attached to a temperature controller 20of the present invention. The temperature controller 20 may also bedefined as a generator. The probe 16 has a thermal element 22 attachedto a probe tip 24. The thermal element 22 provides a means of alteringthe temperature of tissue by heating or cooling. The thermal element 22includes any of the following: a transducer that delivers RF energy tothe tissue, a resistive heating element that delivers thermal energy tothe tissue, or a cooling element. Examples of probes and energy deliveryare set forth in greater detail in U.S. Pat. No. 5,458,596 to Lax et al.which is incorporated herein by reference. The cooling element includesa means for cooling with liquid nitrogen, or a Peltier cell. Atemperature sensor 26, such as a thermocouple, senses the surroundingtemperature. The controller 20 receives the sensed temperature from thetemperature sensor 26 and controls the amount of power that is suppliedto the thermal element 22 to change the temperature of the probe tip 24or to change the temperature of the tissue such as during the deliveryof RF energy to the tissue.

[0031] In a preferred embodiment, the temperature controller 20 is partof a medical system used by physicians to adjust thermal energy to softtissue. To set a target temperature, a physician activates a control 28,such as a knob or a digital switch, on the controller 20. The targettemperature is displayed on a display 30 in degrees Celsius. To selectthe operating characteristics of the controller, the physician adjusts amultiposition switch 32, such as a thumbwheel switch. The operatingcharacteristics are determined by the type of probe 16 and the type oftissue subject to thermal therapy. In other words, each switch positionis associated with a probe and tissue combination. The physician mayobtain the desired operating characteristics, and therefore switchposition, from the manufacturer of the controller 20. Such informationmay be included in the instructions for use (IFU). In this way thephysician can set both the temperature and operating characteristics fordifferent probes.

[0032]FIG. 2 illustrates the controller 20 in more detail. A processor34 communicates with a memory 36 the control 28, the display 30, themultiposition switch 32, and a power control circuit 38 which controls apower source 40 which is attached to the probe. The processor 34includes a microprocessor and peripheral ports that attach to thecontrol 28, display 30, the multiposition switch 32 and the powercontrol circuit 38. The memory 36 includes semiconductor memory. In analternate embodiment, the memory 36 includes disk memory.

[0033] The memory 36 stores a PID_Temperature_Control procedure 42 and aPID_generation procedure 43, which will be described below, sets ofprobe settings, Probe_Settings_1 to Probe_Settings_N, 44 to 46,respectively, a Temperature Profile 47, and a Switch Setting Table 48.An exemplary probe setting 46 includes a proportional gain factor Kp, anintegral gain factor Ki and a derivative gain factor Kd. In addition,the exemplary probe setting 46 may also include a default targettemperature and a default maximum power value. The processor 34 executesthe PID_Temperature_Control procedure 42 to control the probetemperature using a PID control methodology that is implemented in thePID_generation procedure 43.

[0034] Table 1 below shows a preferred set of gain settings. TABLE 1Gain Settings Proportional Gain Integral Gain Derivative Gain Gain SetKp Ki Kd A 0.031 0.008 0.008 B 0.063 0.016 0.016 C 0.125 0.031 0.031 D0.250 0.125 0.063 E 0.500 0.250 0.125

[0035] The higher gain settings (D and E) are beneficial in anapplication in which the physician is heating a large area of tissue andmust move the probe across the tissue. A greater degree of temperatureoscillation may be tolerated due to the larger mass of tissue availableto absorb the variations.

[0036] The lower gain settings (A, B and C) are beneficial in anapplication where the probe is stationary for long periods of time andthe temperature is varied slowly, over minutes. The lower gain settingsprovide more precise temperature control.

[0037] The memory of FIG. 2 also stores a Task_scheduler 49 a, aSet_target_temperature procedure 49 b, a PID_control procedure 49 c anda target_temperature 49 d which will be explained below with referenceto FIGS. 12, 13 and 14.

[0038] In FIG. 3, the switch setting table 48 associates eachmultiposition switch setting with a set of probe settings. Table 2,below, shows the exemplary switch settings of table 48 of FIG. 3. Table2 summarizes the relationship between various switch positions, thedefault temperature, the default maximum output power, gain settings andprobe type. TABLE 2 Switch Settings Default Default Switch TemperatureMaximum Gain Set Position (° C.) Power (W) (See Table 1) Probe Type  055 50 C small  1 55 40 C small  2 55 30 C small  3 55 20 C small  4 6730 C large  5 67 40 C large  6 67 50 C large  7 60 30 C large  8 60 40 Clarge  9 60 50 C large 10 55 20 D small 11 55 30 D small 12 67 40 Dlarge 13 67 50 D large 14 80 40 D large 15 80 50 D large

[0039] In FIG. 4, a hardware implementation of one embodiment of aproportional-integral-derivative (PID) temperature control isillustrated, in which block 50 identifies the components of a hardwareimplementation which accomplishes the method of the present invention.For ease of illustration, the invention will be described with respectto a hardware implementation. A person skilled in the art willappreciate that a software implementation may also be used based on thedisclosure herein. In a preferred embodiment, the temperature controlblock 50 is implemented in software in the PID_Temperature_Controlprocedure 42. The hardware implementation and various embodiments willfirst be discussed, followed by a discussion of the software usingflowcharts.

[0040] On the controller, the physician sets the temperature using thecontrol 28 with associated circuitry which outputs a digital targettemperature signal. The digital target temperature signal is multipliedby a constant gain value, Ks, by amplifier 52. The constant gain valueis typically ten.

[0041] During operation, the probe tip 24 alters the temperature of thetissue 56. The thermocouple 26 senses the surrounding change intemperature and outputs an analog sensed temperature signalcorresponding to the sensed temperature. An analog-to-digital (A/D)converter 58 converts the analog sensed temperature signal to a digitalsensed temperature value. The A/D converter 58 is also calibrated tomultiply the sensed temperature signal by a predetermined value, such asten to match the temperature signal.

[0042] A first summer 60 subtracts the digital sensed temperature valuefrom the digital target temperature value to generate an error value orerror signal, e(t).

[0043] A PID generation block 61 generates three signals or values—aproportional value, an integral value and a derivative value. In thesoftware implementation, the PID generation block 61 is implementedusing the PID_generation procedure 43 of FIG. 2.

[0044] To generate a proportional signal or value, a first amplifier 62multiplies the error value by the proportional gain factor Kp.

[0045] To generate the integral value or signal, a second summer 64subtracts an anti-integral windup signal from the error signal andsupplies its output via a switch 66 to an integrator 68 which integratesthe adjusted error value, as represented by the 1/s Laplace transform,to generate an intermediate value or signal. In a digitalimplementation, the integrator 68 can use any of the followingwell-known algorithms including the trapezoidal, Euler, rectangular andRunge-Kutta algorithms. A second amplifier 70 multiplies theintermediate value by the integral gain factor Ki to generate theintegral value.

[0046] To generate the derivative value, the derivative unit 72 appliesa transfer function to the sensed temperature value to generate anintermediate derivative signal or value. A third amplifier 74 multipliesthe intermediate derivative signal or value by the derivative gainfactor Kd. The transfer function will be discussed in detail below andis represented as a Laplace transform as follows:$\frac{- s}{{0.25s} + 1}$

[0047] A third summer 76 adds the proportional value, the integral valueand the derivative value to generate a PID control value or signal.

[0048] According to a preferred embodiment of the present invention, theproportional gain factor, the integral gain factor, and the derivativegain factor are determined from the multiposition switch setting, thetable and the sets of settings in the memory before starting the PIDcontrol operation. In this way, the PID control function and gains ofthe proportional, integral and derivative values can be customized todifferent types of probes.

[0049] A clamping circuit 78 determines if the PID control value exceedsa predetermined threshold to output an adjusted PID control value. Ifso, the clamping circuit 78 outputs a maximum allowed power value to thepower control circuit 38 to limit or clamp the amount of power suppliedto the probe to prevent overheating. Otherwise, the clamping circuit 78outputs the PID control value. In one embodiment, thePID_Temperature_Control procedure determines the default maximum allowedpower from the default maximum power value of table 48 of FIG. 3. In analternate embodiment, the physician sets the maximum allowed power.

[0050] An antiwindup circuit also helps to limit the amount of power bypreventing the integrator from including large power surges, therebyallowing the integrator to more effectively output a stable steady statevalue and therefore a more stable operating temperature of the probe. Afourth summer 82 subtracts the adjusted PID control value from the PIDcontrol value to generate an antiwindup difference. A fourth amplifier84 multiplies the antiwindup difference by an antiwindup gain factor Kw,typically four, to generate an antiwindup error. The second summer 64subtracts the antiwindup error from the error value.

[0051] Since the adjusted PID control value is typically equal to thePID control value, the antiwindup difference is typically zero and theerror value supplied to the integrator 68 is not affected. When the PIDcontrol value is large, for example when power is first turned on, thePID control value may exceed the maximum allowable power and the PIDcontrol value will be clamped. In this case the antiwindup differencewill be greater than zero and a positive value will be supplied to thepositive input of the second summer 64 to reduce the magnitude of theerror value supplied to the integrator, thereby reducing the effect oflarge surges.

[0052] The physician uses a foot switch 86 to control the amount ofpower that is supplied to the probe. The foot switch power control 86controls the switch position of switches 66 and 38. When the foot switchpower control 86 is not engaged, a zero value is supplied to theintegrator 68 via the zero block 92 at a first switch position.Similarly, another zero block 94 is used by the power control circuit 38such that no power is output to the probe. When the foot switch powercontrol 86 is engaged, switch 66 changes to a second switch position andallows the output of the second summer 64 to flow to the integrator 68.In addition, switch 38 changes to a second switch position and allowsthe output control value to flow from the clamping circuit 78 to theprobe.

[0053] In FIG. 5, the derivative unit 72 implementing the transferfunction described above is shown. The derivative unit 72 receives aninput signal X and outputs a value Y. A fifth amplifier 96 multipliesthe input signal X by a value A0. The derivative unit 72 includes anintegrator 98 that dampens the effect of the derivative function therebyreducing the sensitivity of the derivative unit 72 to large changes inthe input signal and to noise. In a digital implementation, theintegrator 98 can use any of the following well-known algorithmsincluding the trapezoidal, Euler, rectangular and Runge-Kuttaalgorithms. At power on, the integrator 98 output is initialized tozero. A sixth amplifier 100 multiplies the integrator output by A0 togenerate a modified integrated signal. A fifth summer 102 subtracts themodified integrated signal from the multiplied input signal. A seventhamplifier 104 multiplies the output of the fifth summer 102 by B1 togenerate the intermediate integrated value. In a preferred embodiment,A0 is equal to four and B1 is equal to one.

[0054]FIG. 6 is similar to FIG. 4, except that the antiwindup functionis implemented differently. This implementation uses the antiwindupdifference as a switch to stop the integrator from integrating, therebyresulting in an improved steady state operation. When the antiwindupdifference is equal to zero the integrator 68 integrates. When theantiwindup difference is not equal to zero, the integrator 68 stopsintegrating.

[0055] As described above, the fourth summer 82 generates the antiwindupdifference. A comparator 106 compares the antiwindup difference to azero value 107. An inverter 108 inverts the output by the comparator106. In response to the output of the inverter 108 and a signal from thefoot switch, the AND gate 110 generates a position control signal thatcontrols switch 64.

[0056] In particular, when the foot switch is not engaged by thephysician, the foot switch power control signal is a zero value, the ANDgate 110 outputs a digital zero value, and the switch 64 moves to thefirst switch position to output a zero value, thereby preventing theintegrator 68 from integrating.

[0057] When the foot switch is engaged, the foot switch power controlsignal is a digital one value and the AND gate 110 will respond to theantiwindup circuit. When the antiwindup difference is equal to zero, thecomparator 106 outputs a digital zero value which is inverted to adigital one value by inverter 108. Since the inverter 108 is outputtinga digital one value, the AND gate 110 outputs a digital one value andswitch 64 is positioned at the second switch position, as shown in FIG.6, and the integrator 68 integrates the error signal e(t).

[0058] When the antiwindup difference is not equal to zero, theantiwindup difference is a positive value, the comparator 106 outputs adigital one value and the inverter 108 outputs a zero value. In responseto the zero value from the inverter 108, the AND gate 110 outputs adigital zero value and switch 64 is positioned at the first switchposition to output the zero value to the integrator 68, therebypreventing the integrator 68 from integrating.

[0059]FIG. 7 is similar to FIG. 6 except that the error signal e(t) issupplied to the derivative block 72.

[0060]FIG. 8 is a flowchart of the PID_Temperature_Control procedure 42of FIG. 2 that implements the PID control method of FIG. 4. In step 112,sets of probe settings and a table associating the probe settings withswitch settings are provided in the memory, as described above. Each setcorresponds to predetermined operating characteristics for a particularprobe. In step 114, the PID_Temperature_Control procedure 42 receives atarget temperature. The target temperature can be set by the physicianin degrees Celsius. The target temperature value used by the PIDtemperature controller is the temperature set by the physician indegrees Celsius multiplied by a factor, such as ten. In step 116, thePID_Temperature_Control procedure 42 receives a first settingcorresponding to a desired set of operating characteristics from themultiposition switch. In step 118, the PID_Temperature_Control procedure42 selects a particular set of the sets of probe settings in response tothe multiposition switch setting. The particular set has theproportional, integral and derivative gain factors, Kp, Ki and Kd,respectively, as described above, that will be used by thePID_generation procedure. If the physician has not set a targettemperature, the default target temperature that is stored in memory forthe selected switch setting is used. In step 119, thePID_Temperature_Control procedure waits a predetermined amount of timebefore the next sample period. In one embodiment the predeterminedamount of time is equal to twenty milliseconds. In other words, thePID_Temperature_Control procedure samples the sensed temperature valueoutput by the probe every twenty milliseconds. In one implementation,the PID_Temperature_Control procedure uses interrupts to trigger thesample periods. In step 120, a sensed temperature value is received.Similar to the target temperature, the sensed temperature valuerepresents the actual temperature in degrees Celsius and multiplied by afactor of ten. In step 122, an error value is generated by subtractingthe sensed temperature from the target temperature.

[0061] As shown by the dashed lines, steps 124 to 130 are implemented inthe PID_generation procedure 43 of FIG. 2 which is invoked by thePID_Temperature_Control procedure. The PID_generation procedure alsocorresponds to the PID generation block 61 of FIG. 4. In step 124, aproportional value is generated by multiplying the error value by theparticular proportional gain parameter, Kp. In step 126, an integralvalue is generated by subtracting the anti-integral windup value fromthe error value, integrating the resulting value of the subtraction andmultiplying the integrated adjusted error value by the particularintegral gain parameter, Ki. The integrator 68 can be implemented withany of the following well-known algorithms including the trapezoidal,Euler, rectangular and Runge-Kutta algorithms. In step 128, a derivativevalue is generated by applying a derivative transfer function to thesensed temperature value, as described above, and multiplying the resultof the transfer function by the particular derivative gain parameter. Instep 130, an output control signal is generated by summing theproportional value, the integral value and the derivative value.

[0062] In step 132, the output control signal is clamped to apredetermined output value when the output control signal exceeds apredetermined threshold value. The predetermined threshold value is thedefault set power from table 2. The predetermined threshold value can beset by the physician. Alternately, based on the multiposition switchsetting, the default maximum power value stored in one of the tables,described above, is used. In step 134, an amount of power is output tothe thermal element of the probe in response to the output controlsignal, and the process repeats at step 120.

[0063]FIG. 9 is a detailed flowchart of step 128 of FIG. 8 whichgenerates the derivative value. In step 136, the current sensedtemperature value is multiplied by a first constant, A0. In step 138,subtract an integrated output value from the multiplied current sensedtemperature to generate a temporary value. Initially, the integratedoutput value is zero and is modified with each current sensedtemperature reading. In step 140, the temporary value is multiplied by asecond constant, B1, to generate the derivative value. In step 142, anew integrated value is generated based on a previous sensed temperaturevalue and the current sensed temperature value. The integration can beperformed using any of the following well-known algorithms including thetrapezoidal, Euler, rectangular and Runge-Kutta algorithms. The newintegrated value is multiplied by the first constant, A0, to generateanother integrated output value which is used in subsequentcalculations. As described above, preferably, the first constant, A0, isequal to four and the second constant, B1, is equal to one.

[0064] In an alternate embodiment of FIGS. 8 and 9, the error values areinput to the derivative operation instead of the sensed temperaturevalues.

[0065]FIG. 10 is a flowchart of the PID_Temperature_Control procedure 42of FIG. 2 that implements the antiwindup function of FIG. 4. In step152, an antiwindup difference is determined by subtracting a maximumpredetermined clamping value from the output control signal. In step154, an antiwindup adjustment value is generated by multiplying theantiwindup difference by an antiwindup gain factor. In step 156, theantiwindup adjustment value is subtracted from the error value togenerate a modified error value. In step 158, the modified error valueis integrated.

[0066]FIG. 11 is a flowchart of the PID_Temperature_Control procedure 42of FIG. 2 that implements the alternate embodiment of the antiwindupfunction of FIG. 6. In step 162, an antiwindup difference is determinedby subtracting a maximum predetermined clamping value from the outputcontrol signal. In step 164, when the antiwindup difference is not zero,the procedure stops integrating the error values.

[0067]FIG. 12 is a flowchart of the PID_Temperature_Control procedure 43of FIG. 2 that implements variable temperature setting. Physicians maywant to change the temperature profile depending on the application.When operating on large joints, the physician may want to use the probein a high power mode to heat the probe quickly and maintain the targettemperature. However, when operating on the spine, the physician maywant to use a low power mode with a very controlled temperature and noovershoot.

[0068] In this embodiment of the invention, the physician via themultiposition switch can select a particular temperature profile (47,FIG. 2). The physician also may set a final target temperature. In FIG.12, in step 172, in the PID_Temperature_Control procedure, the selectedswitch position corresponds to a particular temperature profile with aramp parameter at which to ramp up the output temperature. Referringalso to FIG. 13, additional exemplary temperature profiles are shown.Each profile 176, 178 stores a ramp parameter (Ramp 1, Ramp N), gainsettings, and a final target temperature. Referring back to FIG. 12, instep 180, in response to the switch position, the target temperature isinitialized to a starting temperature based on the ramp parameter. Theset of gain factors associated with the ramp parameter are retrieved andloaded into a PID control block for use by the PID_control procedure. Instep 182, the PID_Temperature_Control procedure configures themicroprocessor to generate an interrupt at predetermined intervals,preferably every twenty milliseconds.

[0069] The steps in block 184 are executed in response to the interrupt.In step 186, the target temperature is set using theSet_target_temperature procedure (49 b, FIG. 2). If step 186 is beingexecuted in response to a first interrupt, the target temperature isalready set to the starting temperature. Otherwise, the targettemperature is changed by adding the ramp parameter to the targettemperature if a predetermined amount of time has elasped betweensuccessive target temperature changes. Preferably, the targettemperature is changed every thirty seconds. If the sum of the rampparameter and the target temperature exceeds the final targettemperature, then the target temperature is set to the final targettemperature.

[0070] In step 188, the PID_control procedure (49 c, FIG. 2) is executedto control the temperature of the probe. The PID_control procedure isexecuted at each interrupt, every twenty milliseconds. The PID_controlprocedure will be shown in further detail in FIG. 14.

[0071] In step 190, the PID_Temperature_Control procedure waits for thenext interrupt to occur.

[0072] Preferably, the microprocessor executes a task scheduler (49 a,FIG. 2), such as a round-robin task scheduler, to generate theinterrupts and to execute the Set_target_temperature procedure and thePID_control procedure as tasks. The target temperature is stored in thememory (49 d, FIG. 2) for access by both the Set_target_temperatureprocedure and the PID_control procedure.

[0073] In an alternate embodiment, the Set_target_temperature procedurechanges the gain factors in addition to changing the target temperature.

[0074] For example, for a particular switch position setting, a lowpower application with a very controlled temperature is desired. Basedon the switch position, the PID_Temperature_Control procedure sets aninitial target temperature that is much lower than the final targettemperature. The PID_Temperature_Control procedure also uses thepredetermined set of gain values associated with the particular switchposition setting and the interrupts are configured. In response to theinterrupts, the Set_target_temperature procedure and the PID_controlprocedure are executed every twenty milliseconds.

[0075] After thirty seconds have passed, the Set_target_temperatureprocedure increments the initial target temperature by a predeterminedamount, such as one degree, to generate the next target temperature. Inthis way, the Set_target_temperature procedure increments theintermediate target temperature until the final desired targettemperature is reached. As a result, the temperature of the probe isvery well-controlled and overshoot is substantially avoided.

[0076] In FIG. 14, a flowchart of the PID_Control procedure of step 188of FIG. 12 is shown. The PID_Control procedure uses steps 120-134 ofFIG. 8, which were described above. The antiwindup adjustment of FIG. 10or FIG. 11 can be used with the PID_Control procedure of FIG. 14.

[0077] In this way, a method and apparatus are provided that control thesensed temperature of a probe in a strictly controlled manner to avoidoverdamping and underdamping. In addition, depending on the type ofprobe, the target temperature can be set to increase or decrease thetissue temperature. Therefore, the method and apparatus can control bothhigh temperature and low temperature probes to heat or cool tissue.

[0078] The foregoing description, for purposes of explanation, usedspecific nomenclature to provide a thorough understanding of theinvention. However, it will be apparent to one skilled in the art thatthe specific details are not required in order to practice theinvention. In other instances, well known circuits and devices are shownin block diagram form in order to avoid unnecessary distraction from theunderlying invention. Thus, the foregoing descriptions of specificembodiments of the present invention are presented for purposes ofillustration and description. They are not intended to be exhaustive orto limit the invention to the precise forms disclosed, obviously manymodifications and variations are possible in view of the aboveteachings. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the following claims and theirequivalents.

What is claimed is:
 1. A method for controlling a power output of aprobe connected to a system including controller circuitry to maintain atarget temperature, the probe having a thermal element and a temperaturesensor, the temperature sensor providing a sensed temperature,comprising the steps of: providing, in a memory, at least one set ofprobe settings including at least one gain parameter and correspondingto predetermined operating characteristics for a particular probe;receiving the target temperature; receiving a first probe settingcorresponding to a desired set of operating characteristics for a probe;selecting a set from the at least one set of probe settings in responseto the first probe setting; generating an error signal by comparing thesensed temperature to the target temperature; determining an outputcontrol signal by applying a control function to the error signal, thecontrol function using the at least one gain parameter from the selectedset; and controlling an amount of power output to the thermal element inresponse to the output control signal to maintain the targettemperature.
 2. The method of claim 1 wherein the at least one gainparameter of the at least one set of probe settings includes a setspecific proportional gain factor and a set specific integral gainfactor, and wherein: said step of determining an output control signalincludes the steps of: generating a proportional signal by multiplyingthe error signal by the selected set specific proportional gain factor;generating an integral signal by integrating the error signal andmultiplying the integrated error signal by the selected set specificintegral gain factor; and summing the proportional signal and theintegral signal to generate the output control signal.
 3. The method ofclaim 2 wherein the at least one gain parameter of the at least one setof probe settings includes a set specific derivative gain factor, andwherein: said step of determining the output control signal includes thestep of: generating a derivative signal by applying a derivativefunction to the sensed temperature to generate an intermediate signal,and multiplying the intermediate signal by the selected set specificderivative gain factor, wherein said step of summing also sums thederivative signal to generate the output control signal.
 4. The methodof claim 1 wherein the at least one gain parameter of the at least oneset of probe settings includes a set specific proportional gain factor,a set specific integral gain factor and a set specific derivative gainfactor, and wherein: said step of determining an output control signalincludes the steps of: generating a proportional signal by multiplyingthe error signal by the selected set specific proportional gain factor;generating an integral signal by integrating the error signal andmultiplying the integrated error signal by the selected set specificintegral gain factor; generating a derivative signal by comparing thesensed temperature to a previous sensed temperature and applying aderivative function to generate an intermediate signal and multiplyingthe intermediate signal by the selected set specific derivative gainfactor; and summing the proportional signal, the integral signal and thederivative signal to generate the output control signal.
 5. The methodof claim 1 further comprising the step of: limiting the output controlsignal to a predetermined output value when the output control signalexceeds a predetermined threshold.
 6. The method of claim 2 furthercomprising the step of: when the predetermined output value exceeds apredetermined threshold value, disabling said step of generating theintegral signal.
 7. The method of claim 2 further comprising the stepsof: determining an antiwindup difference by subtracting a maximumpredetermined output control value from the output control signal;generating an antiwindup adjustment signal by multiplying the antiwindupdifference by an antiwindup gain factor; and generating a modified errorsignal by subtracting the antiwindup adjustment signal from the errorsignal, wherein said step of generating an integral signal integratesthe modified error signal by the integral gain factor.
 8. The method ofclaim 1 further comprising the steps of: receiving a ramp parametercorresponding to a particular profile at which to ramp up the outputpower; and changing the target temperature in response to the rampparameter.
 9. The method of claim 8 further comprising the steps of:receiving a switch setting corresponding to a particular profile atwhich to ramp up the output power; and changing the target temperaturein response to the switch setting.
 10. The method of claim 9 whereinsaid step of selecting the set from the at least one set of probesettings selects a first set of probe settings in response to the switchsetting, and further comprising the steps of: determining anintermediate target temperature in response to the switch setting; andselecting a second set of probe settings in response to the switchsetting when the intermediate target temperature is reached.
 11. Themethod of claim 1 wherein the at least one gain parameter of the atleast one set of probe settings includes a set specific derivative gainfactor, and further comprising the step of: generating a derivativesignal by multiplying the sensed temperature by a first predeterminedsignal, subtracting a temporary integral signal generated from aprevious sensed temperature to generate an intermediate signal, andmultiplying the intermediate signal by the selected set specificderivative gain factor, wherein said step of summing also sums thederivative signal to generate the output control signal.
 12. The methodof claim 1 wherein the at least one set of probe settings includes a setspecific default target temperature and a set specific default maximumoutput power, and wherein said step of receiving the target temperaturesets the target temperature to the selected set specific default targettemperature, and wherein said step of controlling the amount of poweroutput to the thermal element clamps the power output to the selectedset specific default maximum output power.
 13. A method for controllinga power output of a probe connected to a system including controllercircuitry to maintain a target temperature, the probe having a thermalelement and a temperature sensor, the temperature sensor providing asensed temperature, comprising the steps of: providing, in a memory, atleast one set of probe settings including at least one gain parameterand corresponding to predetermined operating characteristics for aparticular probe, the at least one gain parameter including a setspecific proportional gain factor, a set specific integral gain factorand a set specific derivative gain factor; receiving the targettemperature; receiving a first probe setting corresponding to a desiredset of operating characteristics for a probe; selecting a set from theat least one set of probe settings in response to the first probesetting; receiving a sensed temperature; generating an error signal bycomparing the sensed temperature to the target temperature; generating aproportional signal by multiplying the error signal by the selected setspecific proportional gain factor; generating an integral signal byintegrating the error signal and multiplying the integrated error signalby the selected set specific integral gain factor; generating aderivative signal by comparing the sensed temperature to a previoussensed temperature and applying a derivative function to generate anintermediate signal and multiplying the intermediate signal by theselected set specific derivative gain factor; summing the proportionalsignal, the integral signal and the derivative signal to generate theoutput control signal; controlling an amount of power output to thethermal element in response to the output control signal to maintain thetarget temperature; determining an antiwindup difference by subtractinga maximum predetermined output control value from the output controlsignal; generating an antiwindup adjustment signal by multiplying theantiwindup difference by an antiwindup gain factor; and generating amodified error signal by subtracting the antiwindup adjustment signalfrom the error signal, wherein said step of generating the integralsignal integrates the modified error signal by the integral gain factor.14. A system for controlling the power output of a probe connected tothe system to maintain a temperature, the probe having a thermalelement, and a temperature sensor providing a sensed temperature,comprising: a processor; and a memory storing at least one set of probesettings, the at least one set including at least one gain parameter andcorresponding to a predetermined operating characteristics for aparticular probe, the memory also storing instructions that cause theprocessor to: receive a target temperature value; receive a first probesetting corresponding to a desired set of operating characteristics fora probe; select a set of the probe settings in response to the firstprobe setting; generate an error value by comparing the sensedtemperature value to the target temperature value; and determine anoutput control value by applying a control function to the error value,the control function using the at least one gain parameter from theselected set whereby an amount of power is output to the thermal elementin response to the output control value to maintain the temperature. 15.The system of claim 14 wherein the at least one gain parameter of thesets of probe settings includes a set specific proportional gain factorand a set specific integral gain factor, and wherein: the instructionsthat determine the output control value includes instructions to:generate a proportional value by multiplying the error value by theselected set specific proportional gain factor; generate an integralsignal by integrating the error value and multiplying the integratederror value by the selected set specific integral gain factor; generatea derivative value by comparing the sensed temperature value to aprevious sensed temperature value and applying a derivative function togenerate an intermediate value and multiplying the intermediate value bythe selected set specific derivative gain factor; and sum theproportional value, the integral value and the derivative value togenerate the output control value.
 16. The system of claim 14 whereinthe memory further includes instructions to limit the output controlvalue to a predetermined output value when the output control valueexceeds a predetermined threshold.
 17. The system of claim 14 whereinthe memory further includes instructions to: receive a switch settingcorresponding to a particular profile at which to ramp up the outputpower; and change the target temperature in response to the switchsetting.
 18. A computer program product for controlling a power level ofa probe connected to a medical device computer system, the computerprogram product for use in conjunction with the computer system, thecomputer program product comprising a computer readable storage mediumand a computer program mechanism embedded therein, the computer programmechanism comprising: at least one set of probe settings including atleast one gain parameter and corresponding to a predetermined operatingcharacteristics for a particular probe; a first set of instructions thatreceive a target temperature value; a second set of instructions thatreceive a first probe setting corresponding to a desired set ofoperating characteristics for a probe; a third set of instructions thatselect a set from the at least one set of probe settings in response tothe first probe setting; a fourth set of instructions that receive asensed temperature value; a fifth set of instructions that generate anerror value by comparing the sensed temperature value to the targettemperature value; a sixth set of instructions that determine an outputcontrol value by applying a control function to the error value, thecontrol function using the at least one gain parameter from the selectedset; and a seventh set of instructions that cause an amount of power tobe output to a thermal element in response to the output control value.19. The computer program product of claim 18 wherein the at least onegain parameter of the at least one set of probe settings includes a setspecific proportional gain factor and a set specific integral gainfactor, and wherein: the instructions that determine the output controlvalue include instructions to: generate a proportional value bymultiplying the error value by the selected set specific proportionalgain factor; generate an integral signal by integrating the error valueand multiplying the integrated error value by the selected set specificintegral gain factor; generate a derivative value by comparing thesensed temperature value to a previous sensed temperature value andapplying a derivative function to generate an intermediate value andmultiplying the intermediate value by a selected set specific derivativegain factor; and sum the proportional value, the integral value and thederivative value to generate the output control value.
 20. The computerprogram product of claim 18 wherein the computer program mechanismfurther includes instructions to limit the output control value to apredetermined output value when the output control value exceeds apredetermined threshold.
 21. The computer program product of claim 18wherein the computer program mechanism further includes instructions to:receive a switch setting corresponding to a particular profile at whichto ramp up the output power; and change the target temperature value inresponse to the switch setting.